It is known that kidney transplants are spontaneously accepted across selected fully MHC-disparate barriers in mice, kidney allograft tolerance occurs spontaneously, albeit rarely, in humans, and co-transplantation of a kidney with a heart allograft in humans prolongs survival of the heart graft. The mechanisms underlying these kidney-dependent effects are not known and if better understood, could potentially be exploited to improve transplant outcomes and induce robust allograft tolerance in humans. Our new published and preliminary data support the provocative and intriguing concept that erythropoietin (EPO), a hormone produced predominantly by the kidney in adults, plays an unanticipated role in kidney allograft tolerance. Expanding beyond EPO's established role in erythrocyte development, our new data demonstrate that EPO inhibits alloreactive, conventional T cell immunity and augments regulatory T cell (Treg) induction, function and stability. The preliminary data also identify molecular mechanisms that link EPO to these effects. Our findings support the following hypothesis to be tested in this project as part of the P01: kidney-produced EPO directly and locally inhibits alloreactive nave and memory T cells, and simultaneously induces and maintains stability of donor- specific Treg, together facilitating kidney transplant survival and tolerance. We will test this hypothesis by determining the effects of kidney allograft-derived EPO on murine alloimmunity and allograft survival, deciphering the mechanisms through which EPO selectively inhibits conventional alloreactive T cells, and assessing mechanisms through which EPO promotes Treg induction and stability. The proposed work will define the role of EPO as a mediator of kidney allograft tolerance and will delineate cellular and molecular mechanisms underlying EPO's effects on conventional T cells and Treg. The insights derived from our proposed studies will synergize with and help to guide studies to be performed by our collaborators in projects 1 and 2 on Kidney Induced Cardiac Allograft Tolerance within the program project. In addition to deciphering mechanisms, the studies will provide preclinical data on the utility of EPO as a therapeutic agent for improving graft survival in animals, findings that could potentially be translated to human transplant recipients.